The present invention relates to a light branching and coupling device for performing light distribution or coupling in optical fiber communication, or the like.
Conventionally, a light branching and coupling device as shown in FIG. 16 is used for distributing light advancing in an optical fiber.
The light branching and coupling device is formed by thermally fusing light branching and coupling fiber portions 152a of a pair of optical fibers 152, 152 each of which has a core 150 coated with a clad 151.
In such a light branching and coupling device, as shown in FIG. 17, the critical angle .theta. of light 160 advancing in the optical fiber 152 with respect to the fiber optical axis becomes small if the numerical aperture (NA) of the optical fiber 152 is set to be small. As a result, the light 160 becomes difficult to go into and out of the boundary surface between the light branching and coupling fiber portions 152a, 152a. There arises a problem that the distributing performance of the light branching and coupling device becomes poor.
Therefore, as shown in FIG. 16, when the numerical aperture (NA) of the optical fiber 152 is set to be large to thereby increase the critical angle .theta. of light 160 so that the light 160 can easily go into and out of the boundary surface between the light branching and coupling fiber portions 152a, 152a, the distributing performance of the light branching and coupling device is improved.
On the other hand, from the point of view of transmitting performance of light 160 in the light transmission fiber portion 152b as a region other than the light branching and coupling fiber portion 152a of the optical fiber 152, the critical angle .theta. of light 160 increases if the numerical aperture of the optical fiber 152 is set to be large. Accordingly, the path of the light 160 also increases if the number of times of reflection of light 160 in the optical fiber 152 increases. There arises a problem that the loss of light 160 and the distortion of waveform increase.
Therefore, to prevent the loss of the light 160 and the distortion of waveform and attain widening the range of light from the point of view of the light transmitting performance, the numerical aperture is preferably set to be small to thereby reduce the critical reflection angle .theta. of the light 160.
As described above, it is necessary that the numerical aperture of the optical fiber is set to be large in order to improve the distributing performance of the light branching and coupling device whereas it is necessary to set the numerical aperture of the optical fiber to be small in order to improve the light transmitting performance of the optical fiber per se.
It is however extremely difficult to obtain a light branching and coupling device which satisfies such antinomical requirements.